docs: def dict_delta(old, new):
      """Return a dictionary containing the items of ``new`` that are either
      absent from ``old`` or whose values are different; as well as the
      value ``None`` for those keys that are present in ``old``, but
      absent from ``new``.
  
      Useful for describing changes between two versions of a dict.
  
      """
      r = {}
      oldkeys = set(old.keys())
      newkeys = set(new.keys())
      r.update((k, new[k]) for k in newkeys.difference(oldkeys))
      r.update((k, None) for k in oldkeys.difference(newkeys))
      for k in oldkeys.intersection(newkeys):
          if old[k] != new[k]:
              r[k] = new[k]
      return r